feat: authentication with local accounts, sessions, CSRF, RBAC, and OIDC PKCE (phase 3)

- argon2id (t=3, m=64MiB, p=2) PHC hashing honoring embedded params
- token-bucket rate limiting (10/15min per IP+email) on login/register
- opaque 32B session tokens in Mongo, 30-day sliding expiry, logout-all
- CSRF double-submit cookie/header on authenticated mutations
- bootstrap admin from env with forced first-login password change
- requireAuth/requireRole middleware with disabled-account enforcement
- OIDC code flow with PKCE: lazy discovery, account linking only on
  verified email, auto-created developer accounts
- unit tests (RBAC matrix, CSRF, password, rate limiter) + integration
  suite covering the full auth matrix incl. an in-test fake IdP

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/auth/token.go

@@ -0,0 +1,18 @@
+package auth
+
+import (
+	"crypto/rand"
+	"encoding/base64"
+)
+
+// NewToken returns 32 bytes of cryptographic randomness, base64url-encoded —
+// used for session tokens, CSRF tokens, and OIDC state.
+func NewToken() string {
+	b := make([]byte, 32)
+	if _, err := rand.Read(b); err != nil {
+		// Same stance as ulid: a process that cannot read crypto/rand must
+		// not mint security tokens.
+		panic("auth: crypto/rand failed: " + err.Error())
+	}
+	return base64.RawURLEncoding.EncodeToString(b)
+}